In published Swedish patent application 73- 08917-9 and in U.S. Pat. No. 3,960,827 there is described a cancer-associated polypeptide antigen (CAPA), the technique for its isolation and its use in cancer diagnosis and in the preparation of antibodies. The CAPA is now commercialized under the designation TPA, which stands for "tissue polypeptide antigen". As is clear from the specification of the above-identified Swedish patent application and U.S. patent, the isolation of the natural antigen is a complicated procedure which, even if resulting in a practically useful product, still involves high production costs and moreover may involve difficulties in the provision of necessary starting materials, such as tumor tissue, etc. Against this background a synthetically prepared antigen would, of course, by very attractive, in view of the possibility of thereby obtaining a product exactly specified as to its composition, said product also being capable of being prepared at a more favorable price.